Modern aircraft are equipped with flight control systems, of electric type.
In particular, the rudder control system is called electric, because the command ordered by the pilot (position of the rudder bar) is transmitted to a flight control computer in the form of an electrical signal. The computer then uses this pilot command to determine, using internal control laws, the control command which is actually applied to the rudder.
Despite these electrical characteristics, the rudder bar remains a complex mechanical member. It notably comprises a set of springs which makes it possible to recreate a sensation that is artificial (because the rudder bar is no longer linked mechanically to the control surfaces of the aircraft). These artificial sensation generation means also comprise a mechanical rudder trim function. This function, implemented by a mechanical trim, enables the pilot to set the zero load rudder bar position. The adjustment is performed by the pilot using buttons situated in the cockpit.
The presence, on an electric rudder control system as described above, of a mechanical rudder trim, which meets the abovementioned needs, presents drawbacks, notably in terms of weight, cost, installation, bulk, maintenance, etc.
To at least partly remedy these drawbacks, it is known practice from the document FR2980453A1 (or US 2013075535A1) to eliminate the mechanical trim function. The trim then becomes purely software, either manual or automatic. This entails adding a bias between the command from the pilot (characterized by the position of the rudder bar) and a piloting laws input command. A position of the rudder bar at neutral can therefore generate a non-zero command.
However, the addition of such a constant software bias between the position of the rudder bar and the laws input command, presents the following drawbacks:
when the pilot deflects the rudder bar on the same side as the software bias in place, he or she reaches the maximum command before having reached the mechanical endstop of the rudder bar. He or she therefore encounters a dead band (without effect on the aircraft) and he or she thus loses awareness of his or her real authority endstop;
when the pilot deflects the rudder bar in the direction opposite to the software bias in place, he or she will never obtain maximum authority over the laws input command, even with the rudder bar at the mechanical endstop. A loss of aircraft control authority therefore occurs in this situation.